The invention relates to a power supply circuit for applying a rapidly varying high voltage to a penetron-type cathode ray tube for a colour display system comprising a first voltage generator for supplying a constant high voltage to the tube's anode, a second voltage generator for supplying the cathode with a voltage varying in magnitude with the different colours, a first transforming unit for converting video signals referenced to a first reference voltage into video signals referenced to the voltage varying in magnitude with the different colours and functioning as second reference voltage, a second transforming unit for converting a supply voltage referenced to the first reference voltage into a supply voltage referenced to the second reference voltage, and a supply unit for deriving from the supply voltage referenced to the second reference voltage the supply voltages needed to process the video signals referenced to the second reference voltage and to display these video signals on the cathode ray tube.
To enhance the clarity of a display, a multicolour tube is employed. In order to obtain good positioning accuracy while considering the relatively short working distance of such a display system, it is desirable to eliminate the need for a shadow-mask. A penetron tube accomplishes this by using two different phosphor layers. With a certain voltage applied to the tube, the electrons primarily excite the first phosphor layer, while with a certain higher voltage applied to the tube the electrons are sufficiently accelerated to penetrate the barrier layer between the two phosphor layers and excite the second phosphor layer. The two voltages applied to the tube light up the screen in respective colours; and intermediate voltages produce other colours. In practice, four-colour displays are in common use, such as those described in U.S. Pat. Nos. 3,780,339 and 4,099,088.
In the four-colour display described in U.S. Pat. No. 3,780,339 the anode of the tube is supplied with a voltage switchable to two values and the cathode with a d.c. voltage which is modulated with a voltage switchable to two values. In this way, using two switches, the four voltage levels can be obtained to display the particular colours.
In the four-colour display described in U.S. Pat. No. 4,099,088, a "split anode" penetron tube is employed, where one anode is supplied with a constant voltage and the other with a voltage switchable to four values.
Many of the colour display systems, using penetron-type cathode ray tubes, experience the remarkable phenomenon that, with sufficiently rapid switching of the voltages sound waves are propagated from the front of the cathode ray tube. These sound waves are of such an intensity that they cause noise pollution in the vicinity of the display. For the purposes of this application, noise pollution is considered to exist at about 50 dBA.
To prevent noise pollution, the power supply circuit comprises a first voltage generator for supplying a constant high voltage to the anode and a second voltage generator for producing a voltage varying in magnitude with the different colours, which is applied to the cathode after being modulated with the video signals to be displayed to cooperate with the anode voltage to effect the display of the video signals in the particular colours.
The reason that the choice of the voltage to be applied to a certain electrode is determinative of the presence or absence of noise pollution is explained as follows. The front of the penetron tube is protected by an implosion screen cemented onto the picture screen with a silicon rubber. At the vacuum side of the picture screen the phosphor layer is provided with a conductive coating, which serves as an electrode (anode). The penetron tube is mounted in a metal frame. This frame and a portion of the implosion screen which is also provided with a conductive coating are connected to earth. The combination of picture screen and implosion screen may be regarded as a capacitor of which the plates are connected together not by a rigid medium, but by a resilient medium (rubber). On supplying the anode with a high voltage alternating at a certain frequency, the alternating force acting on the implosion screen will cause this screen, as one rigid mass, to move harmonically. The noise produced is attributed to this isophase motion of the implosion screen. The noise can be considerably reduced by keeping the anode voltage constant.
Constant anode voltage power supply circuits are known from U.S. Pat. Nos. 3,720,781 and 3,863,097, however thse circuits do not solve the problem that the transforming units in the power supply circuit, as described in the opening paragraph, capacitively couple video signal disturbances which distort the picture on the screen.